A correlative sonographic and histopathologic analysis was performed in 35 postmenopausal women with greater than 5-mm thickening of the endometrium at pelvic sonography. Women undergoing estrogen replacement were excluded from study. Four distinct sonographic patterns were encountered. Pattern 1 consisted of echogenic endometrium with small cysts (endometrial polyp with cystic hyperplasia [n = 9], atrophic endometrium with cystically dilated glands [n = 5], and atrophic endometrium [n = 3] at microscopic examination). Pattern 2 was homogeneous echogenic endometrium (proliferative endometrium[n = 3] and adenomyomatous polyp [n = 1]). Pattern 3 was irregular, inhomogeneous endometrium with ill-defined hypoechoic areas (endometrial carcinoma [n = 5], complex hyperplasia with atypia [n = 1], blood clots [n = 1], and atrophic endometrium with eosinophilic metaplasia [n = 11). Pattern 4 was thin endometrium with fluid in the endometrial cavity (scant atrophic endometrium [n = 6]). Thus, an endometrial thickness of greater than 5 mm in postmenopausal women is associated with a variety of pathologic conditions. Subclassification of sonographic patterns may be helpful in differentiating benign cystic atrophy or cystic endometrial hyperplasia from malignant endometrial lesions.